Large quantities of water and fertilizers are used during irrigation in nursery operations and other horticultural applications where trees and plants are grown in containers. Since the containers hold a limited volume of growing medium, there is a frequent need for watering and fertilizer supply. In the case of water demand, the containers lose water through evapotranspiration, not only from the soil's surface but also through the sides of the containers. In order to enable easy transport of the containers and plants therein, it is highly desirable to keep the volume of growing medium to a minimum, however, at the increased risk of dehydration and with a high demand on the growing medium to supply nutrients to the growing plants.
Overhead sprinkling irrigation is the method most frequently used in Canada and the United States for watering such plants and distributing dissolved fertilizers. The overhead sprinkler technique is not particularly efficient in terms of water use however, and studies have reported losses of up to 90% of the volume of water used. Part of this water loss is from evaporation and part from surface runoff. As a result of evaporation, dissolved compounds such as salt, nutrients, calcium, etc. gradually deposit upon the irrigated surfaces. Irrigation water is also lost through runoff and represents a contamination risk for the soil surface and groundwater. Runoff water contains fertilizers, nutrients, biosides and granular herbicides which are leached from the soil substrate.
In recognition of this problem, regulations have been adopted or are impending in a number of North American or European jurisdiction. The intent of the regulations is to require closed circuit irrigation and fertilization systems with no runoff permitted. A closed system would require recovery and recirculation of water and nutrients, however, at present there are few cost effective options available to nursery producers. A closed system would require installation of water catchment and treatment systems involving holding tanks and filtration systems, together with rigorous monitoring of water quality to regulate the accumulation of salts and contaminants.
Micro-irrigation systems provide another alternative, however, they do not necessary eliminate all runoff due to leaching from the bottom of the containers, Micro-irrigation however, does have the advantage of significantly reducing the volumes of water used. Due to the high installation costs however, micro-irrigation is an expensive alternative and requires strict control on water quality to avoid plugging of the conduits and other components of the system. In addition, micro-irrigation systems are best suited for uniform arrays of similar plants and are not flexible in respect of placement and size of containers used. For these reasons, the application of micro-irrigation has been restricted to production of large trees in containers with a volume greater than 20 liters. Micro-irrigation therefore cannot be considered as a general solution to the problems faced by nursery growers utilizing conventional irrigation systems.
A further alternative used in the prior art is sub-irrigation where plants in pots are placed upon capillary mats which provide water through capillary action into the bottom of a permeable pot. A significant advantage of the sub-irrigation system is that no further equipment is required other than that already at the disposal of producers who generally use overhead sprinkler irrigation systems.
The sprinklers are used to water the plants and any water falling between pots is captured by the capillary mat. The water saturated capillary mat thereafter, provides water on demand through capillary action from the roots and permeable base of the pot resting on the capillary mat. Water quality control is not effected since water quality is of the same nature as required by conventional sprinkler irrigation systems. Control of nutrient concentration is required where evaoporation loss is high. Sub-irrigation allows water to be provided according to the needs of each plant on demand since water is drawn up under capillary action by the roots and permeable pot base resting on the saturated capillary mat.
Therefore, sub-irrigation on capillary mats is a viable alternative that meets the objectives of a closed system namely, elimination of runoff water and environmental risks associated, and efficient use of water and fertilizers over conventional systems. Sub-irrigation on capillary mats generally involves maintaining the capillary mats at or near saturation point in order to provide the plants with a constant supply of water. Water moves from the saturated mat to the pots, soil substrate and roots by capillary ascent.
However, due to several disadvantages, the use of capillary mats has traditionally been restricted to greenhouse applications. For example, the water held by the mat often evaporates from the surface and constitutes a not lose in addition to contributing to deposit of salts, nutrients, etc. on the mat surface itself. In humid greenhouses however, where there is a high relative humidity and absence of air movement, evaporation is not a major problem in contrast to field conditions. In an open field, where a nursery includes large trees, for example, algae and weeds will rapidly grow in the exposed surface of the capillary mat. As well, in an open field the low relative humidity of the air, solar heat radiation and high air movements will result in high evaporative losses. As result therefore, these drawbacks have limited the interest in sub-irrigation practices for outdoor use in nurseries. Although sub-irrigation can eliminate runoff in the environment, the not result is a decrease in water use efficiency due to high evaporative losses.
To address the problem of surface evaporation and accompanying salt accumulation on the capillary mat, and to limit algae growth and weed growth in the capillary mats, prior art systems have included a porous perforated plastic sheet on the top surface of the capillary mat. The perforated sheet allows rain water and irrigation sprinkler water to flow through the perforations into the capillary mat. The perforated sheet also allows water to flow out of the capillary mat into the bottom of permeable pots resting on the mat. While dark colored perforated plastic sheets have been very efficient in preventing algae growth in the capillary mat, they still allow significant evaporation from the surface to the extent that evaporated losses exceed losses experienced through recycling or standard irrigation with no recycling. The stored water surface is directly under the top perforated sheet in such prior art carpets, and evaporative loss through the perforated sheet is prohibitively high.
A further problem with sub-irrigation is the tendency for roots of the plants to grow through holes in the pot and into the mat itself. Roots of some species of shrubs for example, have very vigorous root development, emerge from the pot container and continue to grow into the capillary mat layers. In a commercial operation, this root growth results in damage to the roots when pots are lifted from the mat and further during transport. Decomposition of the roots in the mat has a detrimental effect on the structural and hydraulic properties of the mat.
Conventional responses to this problem include biobarriers which include a herbicide or copper. Copper biobarriers release copper ions into water saturated soil. The copper ions in solution in the soil have a proven inhibitory effect on root apex growth. In the presence of copper ions, the roots cease to grow and branch out. U.S. Pat. No. 5,575,112 describes a copper coated geosynthetic membrane for such an application. A disadvantage of this system is that the hydraulic properties of the copper treated synthetic membrane do not allow an adequate hydraulic connection between the capillary mat and the pot substrate. Although the root growth problem has been solved by such a system, there is significant hydophobic impediment placed between the hydraulic connection of the capillary mat and soil substrate, and the biobarrier becomes unworkable. Copper coated geosynthetics available on the market require use of a resin which contains copper hydroxide. It is believed that the resin creates a hydrophobic bebaviour in the geosynthetic membrane. For subirrigated containers on a capillary mat this characteristic increases the risk of hydraulic rupture between the mat and the substrate.
It is an object of the present invention to provide a copper impregnated biobarrier which has adequate hydraulic properties to facilitate the hydraulic connection between the water bearing mat and substrate held within the pots.
It is a further object of the invention to provide an enhanced capillary mat which will overcome the problem of evaporated losses experienced with prior art capillary mats when used outdoors.